1. Field of the Invention
Embodiments of the present invention generally relate to an electrostatic deflection system used in electron beam systems.
2. Description of the Related Art
An electron beam is a group of electrons that have approximately the same kinetic energy and move in approximately the same direction. Electron beam technologies are used in many fields, such as cathode ray tubes (CRT), lithography, scanning electron microscopes, and welding. Electron beam systems, such as scanning electron microscopes, vector and raster beam lithography systems, usually have an electron beam column configured to deflect an electron beam over a target, e.g., a mask or a wafer.
The electron beam column usually have a deflection system to deflect the electron beam over the target either by scanning it, e.g. in image creation, or by arbitrarily deflecting it, e.g. in a vector lithography system. Electron beam systems usually have an objective lens (or delivery optics) which serves to focus the deflected electron beam on the target. Electron beams are generally deflected by a magnetic or an electric field. An electrostatic deflection system is a system that uses an electric field to deflect the electron beams. Because an electric field is generally faster than an magnetic field in deflecting an electron beam, electrostatic deflection systems are usually used to implement fast deflection and to achieve high throughput in the electron beam systems.
Electron beam systems usually require vertical incidence, i.e. that the principal beam (central beam) hits the target perpendicularly so that changes in target height and small focusing do not result in distortion of the scanned or imaged area. In state-of-the-art systems, this is achieved by placing the center of gravity of the deflection system in the back focal plane of the objective lens.
FIGS. 2A and 2B illustrate a side view and a top view, respectively, of a deflection system 245 and an objective lens 250 of a state-of-the-art electron beam system. The deflection system 245 consists of a quardrupole having two crossed capacitors for deflections in x and y directions respectively. Two electrodes 246 form an x capacitor configured to deflect an electron beam 201 in the x direction. Two electrodes 247 form a y capacitor configured to deflect the electron beam in y direction. Point 241 denotes the center of gravity of both pair of electrodes 246 and 247. The objective lens 250 is disposed such that its back focal point coincides with the point 241. For each pair of electrodes 246 and 247, deflection can be considered occurring at the center of the gravity. Therefore, the deflected electron beams 201 always pass through the back focal point of the objective lens 250 and are further deflected by the objective lens 250 to be perpendicular to a target 255.
In electron beam systems, the deflection process may induce aberrations or otherwise degrade of the focused electron beam causing deterioration of resolution. Such aberrations may be minimized by choosing deflector electrodes shaped in arcs of a circle that cover approximately 120°. FIG. 3 illustrates a pair of electrodes 346 for deflecting an electron beam 301 along x direction. Each of the electrode 346 is curved as an arc of a circle. The angle θ is approximately 120°.
Unfortunately, 120 degree electrostatic deflector electrodes cannot be superimposed. The curved electrodes in FIG. 3 cannot be used in the deflection system 245 of FIGS. 2A and 2B because of geometrical limitations, in this case the wide angle of the type of aberration minimizing deflector electrodes. Therefore, the state-of-the-art electron beam systems cannot simultaneously fulfill minimizing aberration and achieving vertical incidence.
Therefore, a need exists for electrostatic deflection systems that allow minimizing aberration and achieving vertical incidence simultaneously.